Adding shadow to picture

I have a picture that is going to be printed on a 9x9 piece of glass and I want my finally dimension to be a 8 inch circle. I will cut the glass after it has been printed but need to add some filler around the image to get to the 9in. I was thinking of a shadow or misty looking something. You probably will see some of it so it need to look decent. Please if anybody can help, the sell at Shutterfly goes off today( I know what till the last minute)....Thanks Michael